A cell line of neural origin (NG108-15) which carries many opiate receptors is used to study the mechanism of action of morphine and morphine-like peptides (endorphin). These materials exert a receptor mediated dual regulation of adenylate cyclase; a rapid inhibition of activity upon which is superimposed a subsequent increase in the amount of total expressible enzyme activity. This homeostatic mechanism may account for tolerance to and dependence upon the opiates. The enkephalins were found to be much more potent than their presumed precursors, the endorphins as mediators of dual regulation. A new class of opioid peptide, the exorphins has been discovered which is derived from food protein via the normal digestive process. The exorphins may provide a new biochemical link between eating, emotional state and psychiatric illness.